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Evapotranspiration Workshop

Calibration for Historical Crop ET Estimates – A Process. Evapotranspiration Workshop. Ivan A. Walter, P.E. Ivan’s Engineering, Inc. Kc ETstzd vs SCS BC. Kc ET stzd is more accurate than SCS BC Kc -- Generate from measurement at known sites Kc -- Transferrable

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Evapotranspiration Workshop

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  1. Calibration for Historical Crop ET Estimates – A Process Evapotranspiration Workshop Ivan A. Walter, P.E. Ivan’s Engineering, Inc. Ivan A. Walter, P.E.

  2. Kc ETstzd vs SCS BC • Kc ETstzd is more accurate than SCS BC • Kc -- Generate from measurement at known sites • Kc -- Transferrable • ETstzd – ET professional & researchers have accepted • SCS BC – Crop Coefficients derived where??? • SCS BC – Requires local calibration Ivan A. Walter, P.E.

  3. Why Isn’t Kc ETstzd Used • Answers to “Why not?” • SCS BC is well accepted in Colo. Water Court. • Do not have the data for ETstzd • Do not have the Kc’s Ivan A. Walter, P.E.

  4. What is Desirable Result • Just some number for ETc • Accurate daily Temp, Rs, U, & Rh • Accurate daily ETrs & ETos • Accurate monthly ETrs & ETos • Excel files & Programs for analysis Ivan A. Walter, P.E.

  5. Criteria (ASCE Task Committee) • Understandable • Defensible • Simplification without loss of accuracy • Based on & Traceable to Experimental Data Ivan A. Walter, P.E.

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  9. Raw Data >>>>>> ETc • Download data from Web site • QC the data – Select good data • Calc ETrs, ETos, and ETHrgv • Calibration Calc’s with good data • COEFHgrv x ETHrgv = ETrs • COEF1Hgrv x ETHrgv = ETos Ivan A. Walter, P.E.

  10. Begin Here Standardized Reference ET Equation: Ivan A. Walter, P.E.

  11. Begin Here w/ Data from Web Ivan A. Walter, P.E.

  12. Calculate & Analyze & Compare Standardized Reference ET Equation: 1985 Hargreaves Equation: Ivan A. Walter, P.E.

  13. Final Product • Standardized Reference ET values • Short Crop: ETos • Tall Crop: ETrs • Daily values Ivan A. Walter, P.E.

  14. Step 1: XXX COAG.xls • Download data from COAGMet site • QC the data • Identify the Good Data • Fix small gaps & errors • Throw out everything else Ivan A. Walter, P.E.

  15. Step 2: ColCrbt2.exe -- Application • Input data from XXX COAG.xls • Text file XXXEtrData.prn • Calculates Reference ET (daily) • ETrs • ASCE PM Etr, 1982 Kimberly Penman ETr • ETos • ASCE PM ETo, Hargreaves ETo Ivan A. Walter, P.E.

  16. Step 3: Colcrbt2.exe -- Results • Linear Regression Analysis (daily) • Zero intercept • Monthly Ratio: ETrs to ETHgrv • Monthly Ratio: ETos to ETHgrv • Monthly Ratio: ETrs to ETos • Regression Coef & SEE Ivan A. Walter, P.E.

  17. Step 3 Results: Ratio ETrs : ETHrgv Ivan A. Walter, P.E.

  18. Results @ Center: Ratio ETrs : ETHrgv Ivan A. Walter, P.E.

  19. Step 3 Results: Ratio ETos : ETHrgv Ivan A. Walter, P.E.

  20. Step 4: ETRETOH 2.exe • Calculate ETrs & ETos using ETHgrv • Input File • DataT&P.prn from XXX COAG.xls • Latitude, Tmax, Tmin, Tavg • COEFHRGV.dat • Monthly Coefficient all Station (15 max.) Ivan A. Walter, P.E.

  21. Step 4: COEFHGRV.dat Ivan A. Walter, P.E.

  22. Step 4: ETRETOH 2.exe • Calculation of ETrs & ETos using 1985 Hargreaves • ETrs = COEF x ETHgrv (Daily) • ETos = COEF1 x ETHgrv(Daily) Ivan A. Walter, P.E.

  23. Step 4: Final Results • Year, DOY, Month, Day, ETrs,ETos, Precip. Ivan A. Walter, P.E.

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  32. Process -- Historical ETref Estimates • CoAgMet Data • XXX COAG.xls -- QC for Good data • Colcrbt2.exe -- Cal’c Monthly Hgrv Factors • ETRETOH 2.exe - Calculate ETrs & ETos Ivan A. Walter, P.E.

  33. Meadow Grass ET • Review of Methods • Recommendation for Calculation ETc Ivan A. Walter, P.E.

  34. Reported Procedures • SCS –BC Pasture Grass • 10% / 1,000 m. change in elevation • ASCE Manual 70 does not recommend • Pochop Method -- ASAE 1984 • Limited to Alfalfa (9%) & Turf (6%) • Adjusted kt --- thus not true SCS BC • Adjust from elevation Coefficient derived • Wyoming Ivan A. Walter, P.E.

  35. Reported Procedures • Denver Water Board, BC “k” coef. • South Park, Elev. 8,900 ft msl • Standard BC not modified BC • Local calibration • ET calculated as: • Lysimeter water use + Precipitation (100%) • ETirr wtr = ETc - Precipitation Ivan A. Walter, P.E.

  36. Recommend Transferrable Kc • Smith’s Gunnison basin • Hargreaves Monthly Coefficients • Letter Describing included in Materials • Maly & Walter – Kc’s Meadow Grass • Memo Describing process included in Materials Ivan A. Walter, P.E.

  37. Hargreaves Kc’s for Meadow Grass • Multiple Sites -- 5 years of data • May = 1.16 • June = 1.28 • July = 1.04 • August = 0.86 • September = 0.88 Ivan A. Walter, P.E.

  38. Meadow Grass Kc -- Derived using Calibrated Hargreaves Ivan A. Walter, P.E.

  39. Software & Documents Provided • XXX COAG.xls -- QC data • Colcrbt2.exe -- Calc’ Monthly Coef. • ETRETOH 2.exe -- Calc’ ETrs & ETos • Example Input and Output files • Memo: Coefficients for Meadow Grass • Smith Letter: Hargreaves Kc M.Grass • Daily Ra calculation Application Ivan A. Walter, P.E.

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